As someone in the Apple ecosystem, Firefox is not a very compelling alternative. I've certainly tried: Firefox Focus was my main iOS browser for a few years (I know it's Webkit-based) and I've tried switching on desktop as well. On desktop, various Firefox updates would change my settings subtly and nag me about features I had no interest in. On top of that, it was simply a slower browser, both with DOM manipulation…
I don't know what browser you've tried, but it certainly is not Firefox Quantum. That browser in my experience is on par with Chrome's performance.
> Firefox Quantum was a revolution in Firefox development. In 2017, we created a new, lightning fast browser that constantly improves. Firefox Quantum is the Firefox Browser.
[1]: https://www.mozilla.org/en-US/firefox/browsers/quantum/